Quinone epoxides synthesis

Porco, Jr. and coworkers also used this biomimehc domino approach for the synthesis of the quinone epoxide dimer torreyanic acid 7-151 as racemic mixture [67]. This natural product, isolated from the fungus Pestalotiopsis microspora, shows a pronounced cytoxicity to tumor cells. Its retrosynthetic analysis leads to the 2H-pyran 7-152 (Scheme 7.40). As substrate for the domino process, the epoxide... 

The quinizarine quinone epoxide 2 (sec p 2783) reacts with the diene 32 bearing a protected x-D-daunosamine as chiral auxiliary and the resulting adduct 33 has been used in the synthesis of daunomycine15. 

We decided to investigate one final substrate that contained our desired side chain for the intramolecular Diels-Alder cycloaddition as well as a small and removable cyano group. Our synthesis is outlined in Scheme 11. Phenol 53 was alkylated with chloroacetonitrile, then condensed to form 2-cyano benzo-furan 54. Subsequent quatemarization to 56 was accomplished with sodium hydride and a bromocrotcMiate (55) electrophile. Following phenol ether deprotection and reduction of the benzylic ketone with sodium borohydride, we were in a position to evaluate the dearomatization step. Unfortunately, all attempts to access the quinone epoxide 58 under classic or modified Adler-Becker reaction conditions failed. With these results, we closed the book on the second chapter in our vinigrol saga and went back to the drawing board. 

Li C, Johnson RP, Porco JA, Jr., Total synthesis of the quinone epoxide dimer (+)-torreyanic acid application of a biomimetic oxidation/electrocyclization/Diels-Alder dimerization cascade. J. Am. Chem. Soc. 2003 125 5095-5106. 

Sanchez, A. J. Konopelski, J. P. Phenol benzylic epoxide to quinone methide electron reorganization synthesis of ( )-taxodone. J. Org. Chem. 1994, 59, 5445-5452. 

The N-benzylcinchonidinium catalyst 70 was successfully employed by Barrett et al. in the synthesis of (—)-preussomerin G [112]. As a key step the epoxide 72 was obtained from the quinone acetal 71 in 81% yield and with 95% ee in the presence of 10 mol% of the ammonium salt 70 (Scheme 10.15). Adam et al. recently reported the highly enantioselective epoxidation of isoflavones [113]. The best results, i.e. ee up to 98% with essentially quantitative yields, were achieved when... 

In the following scheme, the benzo[Z>]furan core of ( )-frondosin B was built up by the palladium-catalyzed quinone reduction, followed by Lewis acid-mediated benzo[Z>]furan formation <07OL3837>. In the total synthesis of bisabosquals, the core structure of benzo[ ]furan was constructed by an epoxide-ring opening reaction <07T10018>. 

Tt-Allylnickel halides. Billington has reviewed the preparation of these complexes from allylic halides using Ni(CO)4 or Ni(COD)2, and their use in synthesis, mainly of natural products (54 references). These complexes react with a wide range of both aliphatic and aryl bromides or iodides as well as aldehydes, ketones, epoxides, and quinones. One advantage is that both allyl ligands react. They do not react with acid chlorides, esters, ethers, nitriles, or acetals. 

The oxidation and reduction steps in the RAQ/RAHQ cycle are performed in two separate reactors. A bubble column is applied for the oxidation of the RAHQ, during which HP is produced. For the Pd-catalyzed hydrogenation of the quinones, a slurry, fixed-bed or monolith reactor can be used. After the reactor and L/L settler, a diluted H P-containing water-methanol stream is finally obtained. After the epoxidation step, crude PO is separated and the water-methanol mixture is returned to the HP synthesis process, thus realizing an efficient process integration. 

The epoxidation of dienes using MMPP occurs, as expected, at the more electron rich double bond, like those reactions using MCPBA. The example shown in Eq. (7) was one of the stages in a synthesis of the quinone antibiotic frenolicin and used MCPBA in the presence of 5% aqueous sodium hydrogen... 

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